Recently, various standards have been developed for data communication over broadband wireless links. One such standard is set out in the IEEE 802.16 specifications and is commonly known as WiMAX. The specifications include IEEE 802.16-2004, primarily intended for systems having fixed subscriber stations, and an enhanced specification IEEE 802.16e-2005 which among other things provides for mobile subscriber stations. In the following description, the term subscriber station (SS) applies to both fixed and mobile stations (SS/MS).
The entire content of IEEE Std 802.16-2004 “Air Interface for Fixed Broadband Wireless Access Systems” is hereby incorporated by reference. IEEE 802.16 envisages single-hop systems in which the subscriber station communicate directly with a base station within range, the range of a base station defining a “cell”. By deploying multiple base stations at suitable positions within a given geographical area, a contiguous group of cells can be created to form a wide-area network. In this specification, the terms “network” and “system” will be used equivalently.
In systems of the above type, data is communicated by exchange of packets between the subscriber stations and base station whilst a connection (management connection or transport connection) is maintained between them. The direction of transmission of packets from the subscriber station to the base station is the uplink (UL), and the direction from the base station to the subscriber station is the downlink (DL). The packets have a defined format which follows a layered protocol applied to the system and its component radio devices. Protocol layers relevant to packets as such are the so-called physical layer (PHY) and media access layer (MAC). In the IEEE 802.16-2004 specification, these protocol layers form a protocol “stack” as shown in FIG. 1. Incidentally, FIG. 1 also shows interfaces between protocol layers in the form of service access points (SAPs), though these are not relevant to the present invention.
The media access layer is responsible for handling network access, bandwidth allocation, and maintaining connections. This includes controlling access of the BS and SS's to the network on the basis of “frames” which are divided in the time domain into a number of slots. Data is exchanged between the MAC peer entities, in other words, between the subscriber station and base station, in units of a protocol data unit (PDU), the PDU being conveyed across the PHY layer using a number of slots. Thus, a “slot” is a unit of time used for allocating bandwidth. The MAC is divided into sublayers including a security sublayer (see FIG. 1) for allowing authentication, key exchange and encryption of PDUs.
Various physical layer implementations are possible in a IEEE 802.16 network, depending on the available frequency range and application; for example, both a time division duplex (TDD) mode—in which uplink and downlink transmissions are separated in time but may share the same frequency—and a frequency division duplex (FDD) mode—where uplink and downlink transmissions can occur at the same time but on different frequencies—are possible. The PHY layer also defines the transmission technique such as OFDM (orthogonal frequency division multiplexing) or OFDMA (orthogonal frequency division multiple access). At present, OFDMA is of most relevance for multi-hop systems of the kind with which the present invention is concerned. A connection between a base station and subscriber station (more precisely, between MAC layers in those devices—so-called peer entities) is assigned a connection ID (CID) and the base station keeps track of CIDs for managing its active connections.
The subsequent description will refer to the TDD mode by way of example. In TDD, each frame is subdivided into a DL-subframe and an UL-subframe. FIG. 3 shows a TDD frame structure illustrating, within the UL-subframe, a packet format having two parts, a PHY header and a MAC PDU. The MAC PDU in turn consists of a MAC header, an optional payload, and optional error correction code (cyclic redundancy code or CRC). The PHY header includes training sequences, frequency band allocation information, and other information relating to physical layer parameters. Within the MAC PDU, the MAC header normally gives essential parameters for media access, such as the type of PDU, MAC address, and type of MAC signalling etc. The CRC within MAC PDU is optional, and can be used to check the received MAC PDU. The payload within MAC PDU is used to contain the data which the SS wishes to send to the BS, but is also optional. For example, some controlling messages, such as a bandwidth request, or an ACK message, have no payload. The payload could be data from higher layer, or sub-MAC-header, which can give additional MAC information.
FIG. 3 also shows, as part of the UL-subframe, an area (request contention field) which consists of a number of request opportunities, used for contention-based bandwidth requests as discussed below. In the case of the TDD mode, bandwidth is allocated on a timing basis, e.g. by allocating slots within frames for the exclusive use of a particular connection (service flow). Meanwhile, the DL-subframe includes a broadcast control field with a DL-MAP and UL-MAP, by which the BS informs the receiving device of the frame structure. The MAP is a map of bandwidth allocation in the frame and consists of Information Elements (IE) each containing a connection ID. Thus, in a TDD mode network, bandwidth allocation means the allocation of resources (slots) within frames. The DL-MAP and UL-MAP are examples of management messages broadcast by the BS (that is, transmitted to all subscribers). Other management messages include an Uplink Channel Descriptor UCD and Downlink Channel Descriptor DCD (both shown in FIG. 3), and Dynamic Service Request and Response (DS-REQ and -RSP) messages.
The concept of quality of service (QoS) is employed in wireless communication systems for allowing a wide range of services to be provided. Depending upon the kind of service being provided (see below), packets may need to be transmitted with a certain accuracy and/or within a certain time delay or they may be useless, and possibly require re-transmission. Thus, during communication with a subscriber station, the base station allocates a QoS level depending on the type of service requested by the subscriber station and available bandwidth, bearing in mind that the base station typically will be communicating with several subscriber stations simultaneously. The QoS parameters take into account priority of transmission (time delay or latency), accuracy of transmission (error rate) and throughput (data rate).
The BS uses a scheduler (scheduling algorithm) to manage the bandwidth (e.g. slot) allocations for all the currently-active connections, balancing the needs of the various subscribers. That is, each SS has to negotiate only once for network entry, after which it is allocated bandwidth by the BS which, though it may increase or decrease on request from the SS or under other demands on the network, remains assigned to that SS thus keeping the connection active. Each connection has a service class and an associated QoS. The QoS is allocated first during a network entry procedure (connection set-up phase) at the time the subscriber station joins the network, and may be modified subsequently by the subscriber station making a request to the base station whilst the connection is maintained. This may involve assigning additional bandwidth to the connection, perhaps repeatedly, depending on available resources in the network.
The relationship between QoS and CID/SFID is illustrated in FIG. 2. For ease of understanding FIG. 2, it is noted that “service flow” refers to transmission of data in a given direction (uplink or downlink) on a connection having a particular QoS. The QoS of the connection is defined by a service flow identifier (SFID) which has a one-to-one relationship to the connection ID. Strictly speaking, it is the service flow (or the connection) to which bandwidth is allocated, but it is convenient to think of bandwidth being assigned by the BS to the SS involved in the connection.
For example, the IEEE 802.16-2004 specification provides four QoS classes or service levels as follows:
(i) Unsolicited Grant Service (UGS):
This service supports real-time data streams consisting of fixed-size packets issued at periodic intervals, such as voice calls (VoIP), in which the packets cannot be delayed appreciably without making the voice call unintelligible. To support this service with small latency, BS will directly grant bandwidth to SS periodically.
(ii) Real-time Polling Service (rtPS):
This supports real-time data streams consisting of variable-sized packets issued at periodic intervals, such as MPEG video. To support this service type, BS shall provide periodic unicast request opportunities, and the SS can send bandwidth request MAC headers by using these opportunities.
(iii) Non-real-time Polling Service (nrtPS):
A service level intended to support delay-tolerant data streams consisting of variable-sized packets for which a minimum transfer rate is needed, such as FTP (File Transfer Protocol). The BS typically polls nrtPS service connection on an interval on the order of one second or less.
(iv) Best Effort (BE)
This lowest service level is for data streams with no particular service requirements. Packets are handled as and when bandwidth is available, with contention-based CDMA-based bandwidth requests (see below) being used by the SS to obtain bandwidth. That is, only if the bandwidth request is sent to the base station without colliding with competing requests from other SS, is the request granted.
In addition, IEEE802.16e-2005 introduces a further service class which is a combination of UGS and rtPS, as follows:
(v) Extended rtPS (ertPS)
A service level intended to facilitate, for example, voice-over IP (VoIP). To support the QoS demand of this service type, BS shall make either unicast grants in an unsolicited manner or periodic polling for SS. This service level is suitable for real-time service flows that generate variable-size data packets on a periodic basis.
In legacy single hop systems (e.g. 802.16-2004 and 802.16e-2005), each mobile station (MS) or subscriber station (SS) may request bandwidth (BW) from the base station (BS), or BS may grant bandwidth to MS/SS directly, thus sharing the access to radio resources. The method of requesting or allocating bandwidth depends on the service class of the connection, more particularly their QoS demands, but basically four methods can be used as follows.
a) Contention based Bandwidth request: The resources are given on a demand assignment basis. Firstly, the SS will send a CDMA code to BS, and then BS will poll this SS for small amount of bandwidth. The SS will use this polled bandwidth to send a bandwidth request MAC header to BS to apply bandwidth for a specific service. When BS receives this bandwidth request, the BS could grant bandwidth to the specific service, such as the Best Effort service mentioned above. FIG. 5 illustrates a packet format used for such a bandwidth request, and FIG. 6 illustrates the signal flow between the MS and BS in this case.
b) Polling: Polling is the process by which the BS allocates to the Subscriber station bandwidth specifically for the purpose of making bandwidth requests, e.g. SS can use this polled bandwidth for sending bandwidth MAC header.
c) Grants: BS can directly give bandwidth to SS periodically by sending Data Grant Burst IEs. This method has smaller latency.
d) Piggybacked bandwidth request: a extension of (a) in which the SS first uses a contention-based bandwidth request to obtain some initial bandwidth, then sends a specific bandwidth request message (or “piggyback” bandwidth request, in which the bandwidth request information is contained in another message) to the BS to obtain more bandwidth.
The BS and SS will know the QoS parameters and the class of each service flow after creating service flows. QoS parameters (and hence QoS information) include: minimum reserved traffic rate; maximum latency; maximum sustained traffic rate, request/transmission policy; tolerated jitter, traffic priority, and unsolicited polling interval. Not all of these parameters are applicable to every service class.
To support addressing and QoS control, some wireless communication systems put connection identification (CID) into a MAC header. For instance, in WiMAX, the service flow between SS/MS and BS can be created and activated during network entry procedure or by dynamic service flow procedure. As mentioned earlier, a service flow ID (SFID) will be assigned to each existing service flow, and each service flow is also associated to a specific QoS demand. A service flow has at least an SFID and an associated direction. The connection ID (CID) of the transport connection exists only when the service flow is admitted or active. The relationship between SFID and transport CID is unique, which means an SFID shall never be associated with more than one transport ID, and a transport CID shall never be associated with more than one SFID.
FIG. 4 shows a generic MAC header format as specified in IEEE 802.16-2004, including a 16-bit CID. FIG. 5 shows an example of a generic bandwidth request, and FIG. 6 shows the conventional signal flow during bandwidth allocation between a BS and MS in a single-hop system.
In single hop wireless communication systems (e.g. IEEE802.16-2004 and IEEE802.16e-2005 as mentioned above), each subscriber station (SS or MS) can communicate with the base station (BS) directly as illustrated in FIG. 6. Recently, efforts are being made to extend IEEE 802.16 to multi-hop (MR) configurations in which traffic between BS and SS is routed via one or more relay stations (RS), rather than being transmitted directly. FIG. 7 shows an example of such a configuration having two relay stations labelled RS1# and RS2#. In this case, the base station is referred to as an MR-BS (Multi-Hop Relay Base Station) since it has extended functionality to support MR. If the network is modified to support relaying functionality as shown in FIG. 7, normally, the relay station (RS) will relay all the packets from the radio devices (subscriber stations or other relay stations) within its coverage, to the MR-BS. FIG. 8 shows one possible signal flow for bandwidth allocation in such a multi-hop system.
In multi-hop relay (MR) systems with distributed scheduling, the MR-BS (Multi-Hop Relay Base Station) shall allocate the bandwidth for relay uplink, which is used for RS to send data to MR-BS; meanwhile, the RS shall allocate the bandwidth for access uplink, which is used for SS to send data to RS. In other words each RS in the MR system requires its own scheduler for allocating bandwidth to the connections in which it is involved. The bandwidth allocation process becomes considerably involved as shown in FIG. 8.
The BS and SS both know the QoS parameters and the service classes of each service flow after creating service flows. However, the RS doesn't know the QoS information of each created service flow, which introduces inconsistency of bandwidth request procedures over relay uplink and access uplink. In addition, the RS may lack knowledge which it requires for its own scheduler algorithms. These problems may increase latency in the network.
Thus, there is a need to reduce the latency involved in bandwidth allocation within a multi-hop relay wireless communication system. More particularly, there is a need for the RS to know at least some QoS information of all service flows in which it is involved, in order to schedule properly.